1. Field of the Invention
The present invention relates to a piezoelectric resonator equipped with a resonator element which comprises a laminate of a plurality of piezoelectric ceramic layers and a plurality of electrode layers which are alternately laminated one upon the other.
2. Description of the Prior Art
The trend toward higher frequencies of electric signals used in radio communications and electric circuits, has urged the development of filters adapted to electric signals of high frequencies.
In the radio communication, for example, microwaves of around 2 GHz are becoming a main stream and, besides, there is a move toward establishing the standards for obliging the use of high frequencies of not lower than several gigahertz. It has therefore been demanded to provide a filter which is cheap in cost and has high performance to meet high frequencies.
An SAW filter is drawing attention in recent years using an elastic surface wave resonator (SAW resonator) which transmits acoustic waves along the surfaces of a solid. This filter utilizes the resonance of an elastic surface wave and a high-frequency electric field applied across the comb-shaped electrodes formed on the surface of a solid, and features a high selectivity of frequencies, and is widely used as an excellent band-pass filter.
In recent years, there has been proposed a resonator utilizing a thickness longitudinal (extensional) oscillation mode of a thin film that exhibits piezoelectric property. In this resonator, the piezoelectric thin film produces thickness longitudinal oscillation which produces resonance in the direction of thickness of the thin film. This resonator therefore is called bulk elastic wave resonator (BAW resonator).
Referring, for example, to FIG. 18, U.S. Pat. No. 4,320,365 discloses a BAW resonator comprising a substrate 61, a support film 63 formed on the surface of the substrate 61, a buffer layer 65 formed on the support film 63, a first electrode 66 formed on the buffer layer 65, a piezoelectric thin film 67 formed on the first electrode 66, and a pair of second electrodes 68 formed on the piezoelectric thin film 67. In this BAW resonator, an oscillation body (resonator element) is formed by the buffer layer 65, first electrode 66, piezoelectric thin film 67 and second electrode 68. The support film 63 is formed on the upper surface of the substrate 61 so as to cover an oscillation space A formed in the substrate 61, and a portion contacting to the oscillation space A in the support film 63 oscillates due to oscillation of the oscillation body (resonator element).
There has further been proposed an acoustic impedance converter as shown in FIG. 19. This acoustic impedance converter includes a support film 72 formed on the surface of a substrate 71 and, further, includes, formed on the support film 72, an oscillation body (resonator element) comprising a first electrode 74, a piezoelectric thin film 73 formed on the first electrode 74, and a second electrode 75 formed on the piezoelectric thin film 73. The support film 72 is a multi-layer film in which thin layers a and b of two kinds of materials having dissimilar acoustic impedances are laminated in many number one upon the other. The thin layers a and b each have a thickness of one-fourth the wavelength of a standing wave produced by the oscillation (resonator element). In such an acoustic impedance converter, ultrasonic waves are effectively reflected by the layers in the support film toward the oscillation body (resonator element), to suppress the leakage of energy into the substrate 71 and to acoustically insulate the oscillation body (resonator element) from the substrate 71 (W. E. Newell, xe2x80x9cFace-Mounted Piezoelectric Resonatorsxe2x80x9d, Proceeding of the IEEE, pp. 575-581, June, 1965 and U.S. Pat. No. 5,373,268).
The resonance frequency of the BAW resonator varies in inverse proportion to the thickness of the film. By using a thin film as a piezoelectric member, therefore, it is allowed to form a resonator of the GHz band. Besides, since the thin film can be directly formed on a semiconductor substrate such as of Si, GaAs or the like, the BAW resonator is drawing attention as an element that can be integrated.
In either the SAW resonator or the BAW resonator, the resonance frequency varies in reverse proportion to the gap between the electrodes. Therefore, a high frequency is obtained by decreasing the gap between the electrodes. In the SAW resonator, however, resonance of the first degree takes place when the gap between the comb-shaped electrodes is one-fourth the wavelength and, hence, the gap between the electrodes inversely varies into four times of the resonance frequency. In a frequency region in excess of 1 GHz, therefore, the gap between the comb-shaped electrodes becomes of the order of submicrons, and it becomes very difficult to form the electrodes (for example, when the comb-type electrodes are to be formed on the board of an LiTaO3 single crystal, the gap between the comb-shaped electrodes becomes about 0.5 microns to obtain a piezoelectric resonance of 2 GHz). Besides, since the gap between the electrodes is of the order of submicrons, lack of resistance against the high-frequency electric power becomes a serious problem in the SAW resonator. In the high-power applications such as the transmission filters, the high-frequency SAW filter of a 2 GHz band has not yet been realized due to the lack of breakdown voltage since the gap between the electrodes is as very small as in the order of submicrons.
In the BAW resonator, on the other hand, resonance of the first degree takes place when the gap between the electrodes is one-half the wavelength, and the gap between the electrodes varies in inverse proportion to the two folds of the resonance frequency. Therefore, when the BAW resonator and the SAW resonator using the piezoelectric member exhibiting the same elastic property and having the same gap between the electrodes, are compared with each other, the BAW resonator makes it possible to obtain a resonance frequency two times as high as that of the SAW resonator. Further, the BAW resonator makes it possible to obtain the same resonance frequency with the gap between the electrodes which is twice as large as that of the SAW resonator and, hence, exhibits excellent resistance against the electric power compared with the SAW resonator.
At present, however, the frequency of the electric signals is sharply increasing, and it has been desired to obtain frequencies higher than high frequencies accomplished by the existing BAW resonators. It has further been desired to provide a filter that exhibits excellent resistance against the electric power in high-frequency regions.
High frequencies are obtained by two methods of either decreasing the thickness of the film or utilizing the resonance of a high degree. When it is attempted to increase the frequency by decreasing the thickness of the film, the gap between the electrodes becomes of the order of submicrons at frequencies of the level of several gigahertz even when the BAW resonator is used as described above, leaving a problem of precision in controlling the film thickness and resistance against the electric power. When the resonance of a high degree is utilized, frequencies which are two folds or three folds as high as the fundamental wave can be utilized while maintaining a given thickness of the film, enabling the resonance of a high frequency to be utilized maintaining a large film thickness. It is therefore possible to provide a resonator which can be used at frequencies of two folds, three folds or four folds as high as that of the conventional resonator that uses the primary standing waves (fundamental waves). In the resonance of a high degree, however, oscillation attenuates with the degree of resonance, resulting in a great reduction in the electro-mechanical coupling coefficient Kt that determines the bandwidth of the filter. Though the frequency could be increased, therefore, a wide-band filter required in the GHz band could not be realized.
Though the piezoelectric resonator such as BAW resonator has been used being incorporated in a piezoelectric oscillation circuit integral with, for example, a capacitor, it becomes necessary to provide space (oscillation permission space) for permitting oscillation between the substrate of the capacitor or the like and the piezoelectric resonator. This prevents the circuit board from being realized in a small size or in a decreased thickness.
The object of the present invention is to provide a piezoelectric resonator having a large electro-mechanical coupling coefficient Kt necessary for realizing a wide-band filter that can be used in a high-frequency band, as well as to provide a filter using the resonator.
Another object of the present invention is to provide a piezoelectric resonator that can be directly joined to the substrate of the capacitor or the circuit board without forming oscillation permission space, that copes with high frequencies, and that is adapted to fabricating the circuit board or the like in a small size and in a decreased thickness.
According to the present invention, there is provided a piezoelectric resonator having a resonator element in or on a substrate, said resonator element comprising a laminate of a plurality of piezoelectric ceramic layers and a plurality of electrode layers which are alternately laminated one upon the other, the piezoelectric ceramic layers neighboring up and down being set to oscillate in opposite phases relative to each other.
According to the present invention, there is further provided a filter having a substrate, a support film formed on the surface of the substrate and a plurality of resonator elements arranged in parallel on the surface of said support film, said resonator elements comprising a laminate of a plurality of piezoelectric ceramic layers and a plurality of electrode layers which are alternately laminated one upon the other, the piezoelectric ceramic layers neighboring up and down being set to oscillate in opposite phases relative to each other.
According to the present invention, further, there is provided a filter having a plurality of resonance elements arranged in parallel in or on the surface of a substrate; wherein
said resonator elements comprise a laminate of a plurality of piezoelectric ceramic layers and a plurality of electrode layers alternately laminated one upon the other in a manner that the piezoelectric ceramic layers neighboring up and down are set to oscillate in opposite phases relative to each other, and a pair of first terminal electrodes and a pair of second terminal electrodes connected to some of the plurality of electrode layers;
the laminate constituting said resonator elements includes the piezoelectric ceramic layers of the same thickness in an odd number which is not smaller than three, and has electrode layers disposed on the outer surface of the uppermost piezoelectric ceramic layer and on the outer surface of the lowermost piezoelectric ceramic layer;
between the pair of first terminal electrodes, one first terminal electrode is connected to the uppermost electrode layer, and the other first terminal electrode is connected to the lowermost electrode layer;
the pair of second terminal electrodes are alternately connected to a plurality of intermediate electrode layers positioned between the uppermost electrode layer and the lowermost electrode layer; and
the amplitude of oscillation occurring in the intermediate piezoelectric ceramic layers between the uppermost piezoelectric ceramic layer and the lowermost piezoelectric ceramic layer is two times as great as the amplitude of oscillation occurring in the uppermost piezoelectric ceramic layer and in the lowermost piezoelectric ceramic layer.
In the piezoelectric resonator element of the present invention, the neighboring piezoelectric ceramic layers oscillate in opposite phases. Therefore, when the resonator element is constituted by a laminate of two piezoelectric ceramic layers each having a thickness t, acoustic standing waves are most efficiently excited having such a degree (second or higher degree) that the half wavelength is just equal to the thickness t. In the conventional BAW resonator comprising a single piezoelectric layer having a thickness 2t, fundamental waves are strongly excited having a half wavelength that is equal to the thickness 2t. Therefore, the piezoelectric resonator of the present invention exhibits a large electro-mechanical coupling coefficient Kt at a resonance frequency two times as high as that of the conventional BAW resonator despite it has piezoelectric ceramic layers (two layers) of a thickness equal to that of the conventional BAW resonator. That is, in the piezoelectric resonator of the present invention, the piezoelectric ceramic layers neighboring up and down undergo the thickness longitudinal (extensional) oscillation in opposite phases, whereby the oscillation of the fundamental mode is suppressed, and the oscillation of a mode of a high degree of not lower than the second degree is strongly excited. Accordingly, the piezoelectric resonator of the present invention utilizing the mode of a high degree of not lower than the second degree, is little affected by spurious caused by the oscillation of the fundamental mode.
When it is attempted to obtain the same resonance frequency as that of the conventional thin-film BAW resonator, the piezoelectric resonator of the present invention makes it possible to increase the film thickness by an amount of utilizing a high-degree mode as compared with the conventional thin-film BAW resonator that utilizes the fundamental mode, contributing to enhancing the degree of freedom for fabrication and, hence, to improving resistance against the electric power (breakdown voltage).
The resonator of the present invention can be driven by setting the direction of polarization to be the same and by applying an electric field in the same direction, or can be driven by setting the direction of polarization to be opposite and by applying an electric field in the same direction. When the resonator is driven by setting the direction of polarization to be opposite and by applying an electric field in the same direction, the film thickness for obtaining the same frequency may be increased to two times as large as that of the conventional thin-film BAW resonator. Besides, the gap between the input electrode and the output electrode at the time of driving can be increased into two folds. Therefore, the intensity of the electric field that is applied can be halved, and the breakdown voltage can be further increased.
Thus, the piezoelectric resonator of the present invention makes it possible to realize a wide GHz-band filter that can be used at frequencies two folds as high as when the SAW resonator or the conventional BAW resonator is used.
In particular, upon setting the thickness of the piezoelectric ceramic layer to be not larger than 2 xcexcm, it is allowed to obtain a strongly excited piezoelectric resonance at a frequency of not lower than 1 GHz and, hence, to realize a favorable thin-film piezoelectric resonator.
Further, when the resonator element is provided on the support film formed on the surface of the substrate, oscillation space is formed in the support film on the side opposite to the surface on where the resonator element is formed, so that the oscillation of the resonator element is little transmitted to the substrate. Thus, there is obtained a piezoelectric resonator having favorable characteristics being little affected by spurious.
Further, the piezoelectric ceramic layer on the outer periphery of the uppermost electrode layer is not polarized, so that the outer peripheral portion acquires a hardness larger than that of the central portion where the energy is confined. Therefore, more energy is confined in the central portion than that of when the outer peripheral portion is polarized in a predetermined direction, and a piezoelectric resonator having favorable characteristics is obtained.
When the piezoelectric ceramic layer is made of a piezoelectric material containing Pb and Ti, the support film is made of silicon nitride, the total thickness of the piezoelectric ceramic layers is denoted tp, the thickness of the support film by ts, and the degree of oscillation of the thickness longitudinal oscillation mode by n, then, it is desired that the ratio ts/tp satisfies:
2.4nxe2x88x925.6xe2x89xa6ts/tpxe2x89xa62.7nxe2x88x924.0
(0 less than ts/tp when the degree n of oscillation is 2).
In this case, the ratio ts/tp of the thickness of the support film to the sum of thicknesses of the piezoelectric ceramic layers is optimized, the electro-mechanical coupling coefficient is maximized in the oscillation mode of any degree, and the electro-mechanical coupling coefficients of oscillation modes of undesired degrees are suppressed. By using such a piezoelectric resonator, it is allowed to obtain a wide-band filter that can be used in a high-frequency band.
Further, when the piezoelectric ceramic layer is made of a piezoelectric material containing Pb and Ti, the support film is made of a diamond film, the total thickness of the piezoelectric ceramic layers is denoted by tp, the thickness of the support film by ts, and the degree of oscillation of the thickness longitudinal oscillation mode by n, then, it is desired that the ratio ts/tp satisfies:
5.4nxe2x88x9212.1xe2x89xa6ts/tpxe2x89xa65.8nxe2x88x928.5
(0 less than ts/tp when the degree n of oscillation is 2).
In this case, too, the ratio ts/tp of the thickness of the support film to the sum of thicknesses of the piezoelectric ceramic layers is optimized, the electro-mechanical coupling coefficient is maximized in the oscillation mode of any degree, and the electro-mechanical coupling coefficients of oscillation modes of undesired degrees are suppressed.
Further, when the piezoelectric resonator of the invention has two piezoelectric ceramic made of a piezoelectric material containing Pb and Ti, operates in the thickness longitudinal oscillation mode, and when the sound velocity (km/s) of the support film is denoted by V, the total thickness of the piezoelectric ceramic layers by tp, and the thickness of the support film by ts, then, it is desired that the ratio ts/tp of satisfies:
0 less than ts/tpxe2x89xa60.2vxe2x88x920.76
in the oscillation mode of the second degree;
0.25vxe2x88x921.08xe2x89xa6ts/tpxe2x89xa60.54vxe2x88x921.84
in the oscillation mode of the third degree; and
0.54vxe2x88x921.75xe2x89xa6ts/tpxe2x89xa60.87vxe2x88x922.86
in the oscillation mode of the fourth degree.
Upon satisfying the above-mentioned conditions, the piezoelectric resonator having two piezoelectric ceramic layers exhibits an optimum ratio ts/tp of the thickness of support film to the sum of thicknesses of the piezoelectric ceramic layers in the oscillation mode of any degree owing to the speed of sound of the support film, maximizing the electro-mechanical coupling coefficient in the oscillation mode of any degree and suppressing the electro-mechanical coupling coefficients of oscillation modes of undesired degrees.
In the piezoelectric resonator of the present invention having a resonator element provided on a support film formed on a substrate, on the surface of the substrate or in the substrate, it is desired that;
the laminate constituting the resonator element includes the piezoelectric ceramic layers of the same thickness in an odd number which is not smaller than three, and has electrode layers disposed on the uppermost layer and on the lowermost layer; and that
the amplitude of oscillation occurring in the intermediate piezoelectric ceramic layers between the uppermost piezoelectric ceramic layer and the lowermost piezoelectric ceramic layer is two times as great as that of oscillation occurring in the uppermost piezoelectric ceramic layer and in the lowermost piezoelectric ceramic layer.
In this piezoelectric resonator, further, it is desired that a pair of first terminal electrodes are connected to the uppermost electrode layer and the lowermost electrode layer (between the pair of first terminal electrodes, one first terminal electrode is connected to the uppermost electrode layer, and the other first terminal electrode is connected to the lowermost electrode layer). Further, the pair of second terminal electrodes are alternately connected to a plurality of intermediate electrode layers positioned between the uppermost electrode layer and the lowermost electrode layer.
In the piezoelectric resonator having such a structure, the piezoelectric ceramic layers neighboring up and down undergo the thickness longitudinal oscillation in opposite phases relative to each other, and the oscillation in the portion except the uppermost piezoelectric ceramic layer and the lowermost piezoelectric ceramic layer, has a phase opposite to, and has an amplitude twice as great as, the oscillation occurring in the uppermost piezoelectric ceramic layer and in the lowermost piezoelectric layer. Therefore, no oscillation occurs on the surface of the resonator element, and the oscillation is confined in the whole resonator element. Accordingly, there is no need to provide oscillation space in the support film on the side opposite to the surface on where the resonator element is formed. There is no need of forming a mirror layer, either, for attenuating the oscillation of the resonator element. In the piezoelectric resonator having a resonator element provided on the surface of the substrate or in the substrate, no oscillation space needs be provided between the substrate and the resonator element. Accordingly, the piezoelectric resonator of the present invention is very advantageous for producing the circuit board and the like in decreased thicknesses and in small sizes.
According to the filter of the present invention in which a plurality of the resonator elements of the above-mentioned structure are arranged in parallel on the support film formed on the surface of the substrate, or directly on the surface of the substrate, or in the substrate, the resonator elements suppress the oscillation of the fundamental mode, and strongly excite oscillation of a mode of a high degree of not lower than the second degree. That is, the present invention makes it possible to obtain a wide-band filter that can be used at high frequencies since the resonator elements are arranged in a plural number in parallel, as compared with the filters using the known piezoelectric resonators. By controlling the ratio ts/tp of the thickness of the support film to the sum of thicknesses of the piezoelectric ceramic layers, further, it is allowed to suppress the electro-mechanical coupling coefficients of undesired oscillation of modes lower than, or higher than, the degree of oscillation that is used and, hence, to obtain a favorable filter that is little affected by spurious.